The present invention relates generally to improvements in hydrophones and more particularly pertains to a new and improved hydrophone preamplifier wherein one or more switches are housed in the preamplifier and controlled remotely without any increase in the number of conductors in the hydrophone cable.
Hydrophones may be thought of an underwater microphones. They convert acoustic signals in water into electrical signals for transmission through a cable to some listening device. Electronic preamplifiers are generally used to boost the signal level and provide an impedance-match to the cable. These preamplifiers are miniaturized and contained in the hydrophone housing.
Many modern hydrophone designs use a cable that consists of only two wires. This reduces the physical bulk and cost of the cable. Since the preamplifier requires DC supply current to operate and it produces an AC audio output signal, it is designed so that these share the common two-wire cable.
It is often necessary to have a relay in a hydrophone preamplifier. For example, a relay is often used to switch the preamplifier input circuit from the actual hydrophone crystal to a "dummy" hydrophone (usually a capacitor). This allows the system noise to be checked at the end of the cable. Sometimes a relay is used to change the gain of the preamplifier or to switch in a special filter. Occasionally one wishes to switch on a special signal source, such as an oscillator for calibration purposes.
In the past, inclusion of such a switch in the hydrophone preamplifier has required the addition of a control wire in the hydrophone cable. Placing a positive or negative voltage on this control wire would cause the relay to change state. Because of the need for this extra control wire, relays could not generally be used in two-wire hydrophone designs. The present invention eliminates this problem.